The present disclosure relates to an image reading device, an image forming apparatus provided therewith, and an optical unit. More particularly, the present disclosure relates to an image reading device including an optical unit movable in a sub scanning direction, an image forming apparatus provided with such an image reading device, and an optical unit.
Some image reading devices designed for incorporation in multifunction peripherals and the like exploiting an electrophotographic process are furnished with a document transport device that feeds sheets of a document successively onto a document stage (glass plate) to allow them to be read and that then, after completion of reading, discharges them off the document stage. Such image reading devices permit document reading by two different methods: by a sheet-through method, in which sheets of a document are read successively while being transported automatically by the document transport device with a document presser kept closed; and by a fixed-document method, in which for each sheet of a document, a document presser is opened and closed to allow it to be placed on a document stage in exchange for any previous one and a scanner unit is moved to read it. Incidentally, in the sheet-through method mentioned first, document reading proceeds while the scanner unit inside the image reading device is held at a predetermined reading position without being moved for scanning. By contrast, in the fixed-document method mentioned later, document reading proceeds while the scanner unit is moved for scanning in the sub scanning direction.
One known planar scanning device (image reading device) permits document reading by the two methods, that is, the sheet-through and fixed-document methods. This planar scanning device is provided with a document stage glass (contact glass) on which a document is placed, an optical unit arranged under the document stage glass and including a scanner unit which is scanned in the sub scanning direction to read an image on the document on the document stage glass, a guide shaft (rail portion) which supports the optical unit such that this is slidable in the sub scanning direction, and a timing belt which enables the optical unit to move in the sub scanning direction. In a bottom part of the optical unit, a bearing that slides relative to the guide shaft is provided
Another known image reading device is provided with, like the planar scanning device mentioned above, an optical unit including a scanner unit (close-contact image sensor unit) which reads an image on a document, a rail portion which supports the optical unit such that this is slidable in the sub scanning direction, and a driving mechanism which enables the optical unit to move in the sub scanning direction, wherein in a bottom part of the optical unit, a bearing that slides relative to the rail portion is provided.
As shown in FIG. 11, in these two image reading devices, a shaft 101 with a circular or oval cross section is used as the rail portion, and in a lower part of the optical unit, a bearing (sliding portion) 102 having a recess 102a with a semi-circular or semi-oval cross section is provided. As shown in FIG. 12, another known sliding portion 103 has a square-cornered U shape so as to make contact with a shaft 101 with a circular cross section on three surfaces.
On the other hand, scanner units can read images by different methods, namely a CCD sensor method employing a CCD (charge-coupled device) sensor and a CIS sensor method employing a CMOS (complementary MOS) sensor.
With the CIS sensor method, the depth of field is small, and this requires the distance from the document to be maintained with high accuracy.
Accordingly, in a CIS sensor method, a scanner unit adopting a CIS sensor method is kept in close contact with the bottom surface of a contact glass. Specifically, a spring for biasing the scanner unit upward is provided, and on the top surface of the scanner unit, a sliding member that slides on the contact glass while keeping contact with it is provided. This helps keep a constant distance between the scanner unit and the document.
However, in cases where, as in the two image reading devices mentioned above, a shaft (rail portion) 101 with a circular or oval cross section supports a bearing (sliding portion) 102 having a recess 102a with a semi-circular or semi-oval cross section or a sliding portion 103 with a square-cornered U shape providing contact on three surfaces, tolerated dimensional errors cause rattling between the shaft 101 and the sliding portion 102 or 103. As a result, when the optical unit is scanned in the sub scanning direction, vibration occurs, inconveniently resulting in lower image reading accuracy.
Conceived to solve the problems discussed above, the present disclosure aims to provide an image reading device that can suppress lowering of image reading accuracy, an image forming apparatus provided with such an image reading device, and an optical unit.